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The historical name pityriasis grouped finely scaling eruptions; its varieties were not all one fungal infection. A useful period list is simple or scalp pityriasis, pityriasis rubra, pityriasis versicolor and pityriasis nigra. The names describe distribution or colour, and classifications differed. In particular, “nigra” was sometimes a very dark form of “versicolor,” sometimes a pigmentation disorder with little true scaling. It cannot safely be translated into one present species of fungus.^[ref]

Simple pityriasis, often called pityriasis capitis on the scalp, gives loose, whitish, bran-like scales with little or variable redness. Microscopy of a scale principally shows flattened keratinised epithelial cells and debris. An oily or inflamed scalp can also contain organisms normally living there; this does not make every case identical to pityriasis versicolor. Present descriptions of dandruff and seborrhoeic dermatitis explain many such appearances, but the historical label cannot retrospectively specify the cause of each case.^[ref]

Pityriasis rubra denoted a conspicuously red, scaling eruption, sometimes extensive. Detached scales are again principally epidermal material. A modern skin biopsy may show an inflammatory pattern, but there is no single microscopical organism that defines the historical category. It must not automatically be renamed pityriasis rubra pilaris, a distinct follicular disorder. An extensive red and peeling skin is a pattern requiring causal diagnosis, rather than proof of one infection.^[ref]

Pityriasis versicolor produces irregular pale, yellowish-brown or brown patches, commonly on the upper anatomical trunk, with a fine surface scale. Here the microscopy is especially discriminating: a superficial scraping, cleared with potassium hydroxide, shows clusters of round yeast forms and short curved hyphae within the stratum corneum. Modern classification places the organisms in Malassezia. The older “Microsporon furfur” terminology should not be read as identification of a modern Microsporum dermatophyte. On a histological section the organisms occupy the horny surface layer and are made conspicuous by suitable fungal stains.^[ref]

Pityriasis nigra calls first for verification of what is actually being described. If it is dark pityriasis versicolor, the same superficial yeast and hyphae may be found; if the lesion is pigmentation without a fungal eruption, this finding cannot be assumed. A contemporary discussion explicitly recorded the overlap of nigra and versicolor and disputes over the significance of the supposed parasite.^[ref] Thus epidermal scales are common to the description, but demonstrable fungal structures particularly distinguish pityriasis versicolor. Examine actual scrapings with a optical microscope, compare several fields and use appropriate controls before attributing a different scaling disorder to a fungus.

A ring-shaped patch is a clinical shape, not one histological disease. The historical category included several named scalp and skin affections whose relationship was disputed. In particular, old claims that alopecia areata was a fungal disease cannot be carried into modern pathology. A contemporary review actually records competing classifications of favus, scalp ringworm, circinate eruptions, beard disease and patchy baldness, and disagreement about whether their fungi were identical.^[ref]

The microscopic distinctions should be organized by the structure infected or injured:

Annular eczema or psoriasis supplies a further clinical mimic: an inflammatory epidermal pattern alone is not evidence of an invading fungus. For an actual microscopic investigation, collect scale from an active margin and affected hairs when hair is involved, mount the material on a microscope slide with potassium hydroxide to clear keratin, and examine under a microscope. A modern biopsy can be examined with Periodic acid-Schiff staining to highlight fungal structures. Distribution within the surface layer, hair shaft, follicle or bulb matters as much as the presence of particles. Fungal culture or molecular identification can resolve species when morphology cannot.^[ref] Distinguish the organism, its anatomical location and the host reaction; do not diagnose all circular lesions as one infection.